1. Field of the Invention
The present invention relates to a non-contact tonometer that calculates an intraocular pressure value from a corneal shape deformation detection signal when air is blown to an eye to be inspected, a control method of the same, and a program to execute the control method.
2. Description of the Related Art
Many of non-contact tonometers have constitution where a piston pushes air in a cylinder outwardly, to pressurize the air in an air chamber, and the pressurized air is ejected from a nozzle. For the driving of the piston, a rotary solenoid is usually used because an initial torque of the rotary solenoid is strong and a stroke thereof is long. Furthermore, control is executed to turn off energize to the rotary solenoid on the basis of a corneal shape deformation detection signal, when a cornea is deformed into a predetermined state that is necessary for tonometry. However, even after the energization to the rotary solenoid is turned off, the piston continues to move for a while by an inertia force. Therefore, the air unnecessary for the tonometry is blown to an eye to be inspected, and hence an excessive burden is imposed on a patient. For the purpose of reducing the unnecessary air as much as possible, it is necessary to decrease a movement of the piston by the inertia force.
To decrease the movement of the piston by the inertia force, in Japanese Patent Application Laid-Open No. H08-000109, a method has been suggested in which when it is detected that a cornea is deformed as much as a predetermined amount, the piston is moved backward. Furthermore, in Japanese Patent Application Laid-Open No. H07-100116, a method has been suggested in which when it is detected that the cornea is deformed as much as the predetermined amount, the movement of the piston is decelerated or stopped by using a brake mechanism such as an electromagnetic coil or a disc brake.
In general, a direction of a force to be applied to the piston by the rotary solenoid is an only pressurizing direction of the air. Therefore, it is difficult for the rotary solenoid alone to execute control so that the piston advancing in the pressurizing direction of the air is rapidly decelerated, stopped or moved backward. Therefore, for implementing the above-mentioned prior art, it is necessary to apply the force from the outside to the piston in an opposite direction to the air pressurizing direction, separately disposing a drive mechanism such as the solenoid or a motor as in Japanese Patent Application Laid-Open No. H07-100116. However, a response time of a usual drive mechanism such as the solenoid or the motor is longer than a time from when the cornea starts to be deformed to the end of the blowing of the air. Therefore, even when there is separately disposed the drive mechanism that applies the force of the opposite direction to the piston on the basis of a corneal shape deformation signal, the drive mechanism starts to operate after the blowing of the air weakens or ends. Therefore, in the above-mentioned prior arts, the time required for the control is excessively long, and hence temporally difficult to reduce the air to be blown.